Variable displacement compressors have been applied to climate control systems for vehicles and were introduced with pneumatic or mechanical control hardware. With the advancement of electronic technologies, solenoids have commonly been used to increase the compressor control range. With the refinement of electronic control valves, compressor clutches are being eliminated. This type of compressor is commonly called a clutchless compressor and is usually an electronically controlled, variable displacement compressor.
The clutchless compressor cannot disengage or re-engage the clutch, hence the compressor is operational whenever the vehicle engine is running. When the climate control system is turned off, the compressor is placed at a minimum displacement. Ideally, the minimum displacement would be zero, but current compressor technology does not allow for such a device. Instead, the compressor must displace some refrigerant at minimum stroke.
Mainstream clutchless compressor technology utilizes a three-port electronic control valve in conjunction with a fixed orifice bleed to determine compressor displacement. In one known method, the electronic control valve regulates the flow of discharge gas to the compressor crank case and the fixed orifice bleed flows refrigerant from the crank case to the suction cavity in a rear head of the compressor. In a second method, the electronic control valve regulates the flow of refrigerant from the compressor crank case to the suction cavity and a fixed orifice bleed defines a pathway for refrigerant to flow from the discharge cavity in the rear head of the compressor to the crank case.